Picosecond Optical Radiation Systems and Methods of Use

ABSTRACT

Methods, systems and apparatus are disclosed for delivery of pulsed treatment radiation by employing a pump radiation source generating picosecond pulses at a first wavelength, and a frequency-shifting resonator having a lasing medium and resonant cavity configured to receive the picosecond pulses from the pump source at the first wavelength and to emit radiation at a second wavelength in response thereto, wherein the resonant cavity of the frequency-shifting resonator has a round trip time shorter than the duration of the picosecond pulses generated by the pump radiation source. Methods, systems and apparatus are also disclosed for providing beam uniformity and a sub-harmonic resonator.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of U.S. ProvisionalApplication No. 61/789,144 filed Mar. 15, 2013 entitled SubnanosecondLaser Systems and Methods of Use and U.S. Provisional Application No.61/891,299 filed on Oct. 15, 2013 entitled Multi-Wavelength OpticalRadiation Sources for Dermatology, the entire contents of which areincorporated by reference herein.

FIELD

The present disclosure relates generally to dermatological systems,methods, and devices and, in particular, to systems, methods, anddevices for applying optical radiation, e.g. laser radiation in thevisible and near infrared wavelengths, to treat tattoos, and otherpigmentation disorders.

BACKGROUND

The use of lasers, as controllable sources of relatively monochromaticand coherent radiation, is becoming increasingly common in diversefields such as telecommunications, data storage and retrieval,entertainment, research, and many others. In the area of medicine, forexample, lasers have proven useful in surgical and cosmetic proceduresin which a precise beam of high energy radiation can cause localizedeffects through photothermal processes (e.g., selectivephotothermolysis) and/or photomechanical processes (e.g., induction ofcavitation bubbles and acoustic shock waves). In dermatologyspecifically, lasers have been used in a wide variety of proceduresincluding hair removal, skin resurfacing, removal of unwanted veins, andthe clearance of both naturally-occurring and artificial skinpigmentations (e.g., birthmarks, port wine stains, and tattoos).

Whereas early laser tattoo removal procedures often utilizednon-selective ablation of tissue at the tattoo site with water servingas the target chromophore, recent procedures have instead utilizedQ-switched lasers capable of producing high-powered, nanosecond pulsesto induce photomechanical breakdown of the tattoo particles themselves.In addition to pulse duration and power, the wavelength of the radiationis also an important parameter in the efficacy of a treatment. Forexample, though alexandrite lasers emitting picosecond pulses atwavelengths between 750 and 760 nm have been found to be especiallyeffective at treating black, blue, and green tattoo pigments (iBrauer etal., “Successful and Rapid Treatment of Blue and Green Tattoo PigmentWith a Novel Picosecond Laser,” Archives of Dermatology, 148(7):820-823(2012)), radiation in the 750-760 nm range is not nearly as effective inremoving red or orange tattoos due to the low absorption coefficient oforange and red tattoo pigments at such wavelengths.

Accordingly, there exists a need for improved methods and apparatus forproducing ultra-short pulses of laser radiation at various wavelengthsfor the treatment of tattoos, pigmented lesions, and other skindisorders.

SUMMARY

Systems, methods, and devices for generating and delivering ultra-shortpulses, e.g., picosecond pulses, of laser radiation at multiplewavelengths with low energy losses are provided herein. It has beenfound, for example, that the picosecond, high power pulses disclosedherein can be particularly effective in removing skin pigmentations, inpart, because the pulses induce mechanical waves (e.g., shock waves andpressure waves) at the target sites that cause greater disruption andbetter clearance of pigment particles. In accordance with variousaspects of the present teachings, the wavelength of the applied pulsescan be selected to match the absorption spectrum of previouslydifficult-to-treat pigments (while nonetheless maintaining theultra-short pulse durations) such that the naturally-occurring andartificial skin pigments can be cleared with a reduced number oftreatments relative to known procedures, thereby providing a system thatcould satisfy a long-felt need in the art. By way of example, themethods and systems disclosed herein can improve the disruption andclearing efficacy of red and orange tattoos by delivering laser pulseshaving a wavelength between about 400 and about 550 nm, where thesepigments exhibit much higher absorption coefficients.

In accordance with various aspects, certain embodiments of theapplicants' teachings relate to an apparatus for delivery of pulsedtreatment radiation. The apparatus can comprise a pump radiation sourcegenerating picosecond pulses at a first wavelength, and awavelength-shifting resonator having a lasing medium and resonant cavityconfigured to receive the picosecond pulses from the pump radiationsource at the first wavelength and to emit radiation at a secondwavelength in response thereto. The resonant cavity of thewavelength-shifting resonator has a round trip time shorter than theduration of the picosecond pulses generated by the pump radiationsource, and in some aspects, the wavelength-shifting resonator can havea round trip time at least 5 times shorter than the duration of thepicosecond pulses generated by the pump radiation source (e.g., at least10 times shorter).

In accordance with various aspects of the present teachings, thewavelength-shifting resonator can have a variety of configurations toproduce the wavelength-shifted picosecond pulses provided herein. By wayof example, the wavelength-shifting resonator can have a cavity lengththat is from about 0.1 millimeters to about 150 millimeters, or fromabout 60 millimeters to about 120 millimeters, or from about 80millimeters to about 100 millimeters. However, in one example, thewavelength-shifting resonator can have a cavity length less than 10millimeters (e.g., a cavity length between 0.1 and 10 millimeters). Invarious aspects, for example, the wavelength-shifting resonator has acavity length between 1 and 8 millimeters. By way of non-limitingexample, the resonator can comprise a neodymium-doped vanadate crystal(Nd:YVO₄) crystal having a length between the input side and the outputside of about 3 mm or a neodymium-doped yttrium-aluminum garnet crystal(Nd:YAG) having a length between the input side and output side of lessthan about 8 mm (e.g., about 6 mm).

As indicated above, the lasing medium can comprise a variety ofmaterials for receiving the pump pulse from the pump radiation source.By way of example, the lasing medium of the wavelength-shiftingresonator can comprise a neodymium-doped crystal, including, a solidstate crystal medium selected from the group of neodymium-dopedyttrium-aluminum garnet (Nd:YAG) crystals, neodymium-doped pervoskite(Nd:YAP or Nd:YAlO₃) crystals, neodymium-doped yttrium-lithium-fluoride(Nd:YAF) crystals, and neodymium-doped, vanadate (Nd:YVO₄) crystals.Moreover, in some aspects, the lasing medium can comprise between about1 and about 3 percent neodymium.

In various aspects, the apparatus can produce polarized opticalradiation. For example, the apparatus can comprise a polarizerconfigured to polarize optical radiation emitted by thewavelength-shifting resonator. Additionally or alternatively, theapparatus can comprise a polarizer embedded within the resonant cavityof the wavelength-shifting resonator. Additionally or alternatively, thelasing medium of the wavelength-shifting resonator can be asubstantially polarizing medium.

In some aspects, the apparatus can further comprise a frequency-doublingwaveguide. By way of example, the frequency-doubling waveguide cancomprise a second harmonic generating, nonlinear crystal material thatcan receive the radiation emitted by the wavelength-shifting resonatorto output a pulse having twice the frequency of the input pulse (i.e.,half the wavelength). In various aspects, the frequency-doublingwaveguide can comprise a lithium triborate (LiB₃O₅) material. In arelated aspect, an amplifier can be disposed between thewavelength-shifting resonator and the frequency-doubling waveguide.

The pump radiation source can in various embodiments have a variety ofconfigurations. By way of example, the pump radiation source can be amode-locked laser, that in some embodiments can comprise a resonator, alasing medium, a Pockels cell and a controller, wherein the controllergenerates a mode-locked pulse by applying a periodic voltage waveform tothe Pockels cell. In some aspects, the mode-locked laser can comprise analexandrite laser configured to produce pulsed laser energy at about 755nm having at least about 100 mJ/pulse (e.g., from about 200 to about 800mJ/pulse). In various aspects, the mode-locked laser can generate pulsedlaser energy having a pulse duration of less than 1000 picoseconds(e.g., about 860 picoseconds).

In accordance with various aspects of the present teachings, theapparatus can further comprise a treatment beam delivery systemconfigured to apply a treatment beam to a patient's skin. In someaspects, the treatment beam can comprise at least one of picosecondpulses from the pump radiation source at the first wavelength,picosecond pulses emitted by the wavelength-shifting resonator at thesecond wavelength, and picosecond pulses at a third wavelength, whereinthe picosecond pulses at the third wavelength are output by afrequency-doubling waveguide that receives the picosecond pulses at thesecond wavelength. In various embodiments, the first wavelength can beabout 755 nm, the second wavelength can be about 1064 nm, and the thirdwavelength can be about 532 nm. Additionally, the apparatus can beoperated so as to enable the selection of the wavelength of the pulse(s)to be applied to a patient's skin through the treatment beam deliverysystem. The apparatus can also control the wavelength-shifting resonatortemperature.

In accordance with various aspects, certain embodiments of theapplicants' teachings relate to a method for shifting the wavelength ofa picosecond optical radiation pulse. The method can comprise generatinga pulse of optical radiation at a first wavelength and having a durationless than 1000 picoseconds, pumping a wavelength-shifting resonator withthe pulse of optical radiation at the first wavelength, thewavelength-shifting resonator comprising a laser crystal with a highabsorption coefficient at the first wavelength, and extracting a pulseof radiation at a second wavelength emitted by the wavelength-shiftingresonator, wherein the pulse at the second wavelength also has aduration of less than 1000 picoseconds. The round trip time of thewavelength-shifting resonator is shorter than the pumping laser pulseduration. For example, the wavelength-shifting resonator can have around trip time at least 10 times shorter than the duration of thepumping pulse.

In various aspects, the method can further comprise one or more ofpolarizing, amplifying, and frequency-doubling the output of thewavelength-shifting resonator. For example, in some aspects, a polarizercan be configured to polarize optical radiation emitted by thewavelength-shifting resonator. Additionally or alternatively, apolarizer can be embedded within the resonant cavity of thewavelength-shifting resonator or the lasing medium of thewavelength-shifting resonator can be a substantially polarizing medium.In some aspects, the pulse of radiation at a second wavelength can betransmitted to a frequency doubling crystal so as to generate a pulsehaving twice the frequency of the input pulse (i.e., half thewavelength).

In accordance with various aspects, certain embodiments of theapplicants' teachings relate to a method for treating tattoos or skinpigmentation disorder using a picosecond optical radiation source. Themethod can comprise employing a pump radiation source to generate apulse of optical radiation at a first wavelength, wherein the pulse hasa duration of less than 1000 picoseconds, and pumping awavelength-shifting resonator with the pulse of optical radiation at thefirst wavelength, the wavelength-shifting resonator comprising a lasercrystal with high absorption coefficient at the first wavelength, andextracting a pulse of radiation at a second wavelength emitted by thewavelength-shifting resonator, wherein the pulse at the secondwavelength also has a duration of less than 1000 picoseconds. Inaccordance with the present teachings, the round trip time of thewavelength-shifting resonator can be shorter than the pumping laserpulse duration. The method can further comprise delivering the pulse ofradiation at the second wavelength to a frequency-doubling waveguide soas to generate a pulse of radiation at a third wavelength, wherein thepulse at the third wavelength also has a duration of less than 1000picoseconds, and directing the pulse at the third wavelength to a tattoopigment or a skin pigmentation target to disrupt the target and promoteclearance thereof. By way of example, the first wavelength can be about755 nm, the second wavelength can be about 1064 nm, and the thirdwavelength can be about 532 nm, and the method can comprise selectingthe wavelength of the pulse(s) to be applied to a patient's skin.

In accordance with various aspects, certain embodiments of theapplicants' teachings relate to a method for removing a tattoo ortreating a skin pigmentation disorder. The method comprises applyingpulses having a duration less than 1000 picoseconds to an area of apatient's skin comprising a tattoo pigment or skin pigmentation so as togenerate photomechanical disruption of the tattoo pigment or skinpigmentation, wherein the pulses have a wavelength in a range of about400 nm to about 550 nm (e.g., about 532 nm). In some aspects, the methodfurther comprises utilizing a Nd:YVO₄ lasing medium to generatepicosecond pulses of radiation having a wavelength of about 1064 nm, andfrequency doubling the picosecond pulses having a wavelength of about1064 nm to generate picosecond pulses having a wavelength of about 532nm.

In one aspect, the disclosure relates to an apparatus for delivery of apulsed treatment radiation such as a laser. The laser having a lightsource, a resonator having a mode lock element, and a lasing medium suchas an active lasing medium. The lasing medium is impinged upon by thelight source. An element is disposed between the light source and thelasing medium, the element enables a substantially uniform gain acrossthe lasing medium. The laser can include a second light source. Thefirst light source and/or the second light source can be a pumpedradiation source such a flash lamp. In one embodiment, the lasing mediumis an alexandrite crystal. The element can be, for example, an aluminarod having a diameter of about 0.063 inches. The element can be at leastone of a deflector, a scattering element, a refractor, a reflector, anabsorber, and a baffle. In one embodiment, element is equidistant fromthe flash lamp and the lasing media. In another embodiment, the elementis disposed on the lasing medium, is disposed on the light source, or isdisposed on both the lasing medium and the light source.

In another aspect, the disclosure relates to an apparatus for deliveryof a pulsed treatment radiation such as a laser. The laser includes alight source and a resonator having a multimode output, a mode lockelement and an astigmatic element disposed inside the resonator. Theastigmatic element can prevent free space propagation modes such asHermites within the multimode output from coupling together. In thisway, beam uniformity is improved with the use of the astigmatic elementcompared to where the astigmatic element is absent. Suitable astigmaticelements can include, for example, at least one of a cylindrical lens,an angled spherical lens, and a prism (e.g., an anamorphic prism).

In another aspect, the disclosure relates to a resonator (e.g., anoscillator) for a mode locked laser having a fundamental frequency whichis the speed of light divided by the round trip optical path length (2L) of the resonator and a mode locking element (e.g., a Pockels cell)that is modulated at a frequency that is less than the fundamentalfrequency. The frequency can be a sub-harmonic (1/n) of the speed oflight (c) divided by the round trip optical path length (2 L) where (n)is whole number greater than 1. The resonator can be employed in anapparatus for delivery of a pulsed treatment radiation, such as a laser,to treat tissue.

In another aspect, the disclosure relates to a resonator (e.g., anoscillator) for a mode locked laser that provides a frequencycorresponding to a fundamental round trip optical path length (2 L) in amode locked resonator and selecting a sub-harmonic optical path lengththat is shortened by dividing the fundamental round trip optical pathlength (2 L) by a sub-harmonic factor (n), which is a whole numbergreater than 1, and the sub-harmonic total path length has n round tripoptical path lengths. The resonator can be employed in a laser to treattissue.

These and other features of the applicants' teachings are set forthherein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled person in the art will understand that the drawings,described below, are for illustration purposes only. The drawings arenot intended to limit the scope of the applicants' teachings in any way.

FIG. 1 , in a schematic diagram, illustrates an exemplary system havinga wavelength-shifting resonator for generating picosecond pulses inaccordance with various aspects of the applicants' teachings.

FIG. 2 , in a schematic diagram, illustrates an exemplary system havinga wavelength-shifting resonator, the system for generating multiplewavelengths of picosecond pulses in accordance with various aspects ofthe applicants' teachings.

FIG. 3 , in a schematic diagram, illustrates an exemplarywavelength-shifting resonator having an embedded polarizer for use inthe systems of FIGS. 1 and 2 in accordance with various aspects of theapplicants' teachings.

FIG. 4 depicts an exemplary output pulse of an Nd:YAG resonator operatedin accordance with various aspects of the applicants' teachings.

FIG. 5 depicts an exemplary output pulse of an Nd:YVO₄ resonatoroperated in accordance with various aspects of the applicants'teachings.

FIG. 6 illustrates an example of the output pulse shape of a shortresonator Nd:YAG laser with a 70% output coupler.

FIG. 7 is a cross-section of a pump chamber in accordance with variousaspects of the applicants' teachings.

FIG. 8 is an axial-view image of the fluorescence of a pumped laser rodin accordance with various aspects of the applicants' teachings.

FIG. 9 is a graph depicting the normalized gain distribution in anunmodified diffuse pump chamber.

FIG. 10 is an axial-view image of the fluorescence of a pumped laser rodin accordance with an embodiment of the present disclosure.

FIG. 11 is a graph depicting the normalized gain distribution in amodified diffuse pump chamber in accordance with an embodiment of thedisclosure.

FIG. 12 is a laser beam profile image of a mode-locked laser using anunmodified pump chamber in accordance with various aspects of theapplicants' teachings.

FIG. 13 is a laser beam profile image of a mode-locked laser using amodified pump chamber in accordance with an embodiment of thedisclosure.

FIG. 14A shows a laser intensity profile that includes the free spacepropagation mode effects caused by two propagating Hermite fields thatare in phase with one another in accordance with various aspects of theapplicants' teachings.

FIG. 14B shows a laser intensity profile when an astigmatic element isintroduced to decouple propagating Hermite fields such that they are notin phase with one another in accordance with various aspects of theapplicants' teachings.

FIG. 15A shows the modulation signal applied to the Pockels cell in apicosecond resonator and the intensity that builds up in the resonatorover time in accordance with various aspects of the applicants'teachings.

FIG. 15B shows the modulation signal applied to the Pockels cell in asub-harmonic picosecond resonator and the intensity that builds up inthe resonator over time in accordance with various aspects of theapplicants' teachings.

FIG. 16 , in a schematic diagram, illustrates an exemplary system forgenerating picosecond pulses in accordance with various aspects of theapplicants' teachings.

FIG. 17 is a plot of the seed pulse generation with a laser capable ofgenerating a sub-harmonic pulse group at 300 mV when the Pockels cellvoltage was low and at 190 mV when the Pockels cell voltage was high inaccordance with various aspects of the applicants' teachings.

DETAILED DESCRIPTION

All technical and scientific terms used herein, unless otherwise definedbelow, are intended to have the same meaning as commonly understood byone of ordinary skill in the art. References to techniques employedherein are intended to refer to the techniques as commonly understood inthe art, including variations on those techniques or substitutions ofequivalent or later-developed techniques which would be apparent to oneof skill in the art. In addition, in order to more clearly and conciselydescribe the claimed subject matter, the following definitions areprovided for certain terms which are used in the specification andappended claims.

The terms “picosecond” or “picosecond pulse,” as used herein, isintended to encompass pulses of optical radiation having durationsranging from 0.1 picoseconds to 1000 picoseconds, preferably less than1000 picoseconds. e.g., less than 900 picoseconds, less than 800picoseconds or less than 700 picoseconds. For non-square pulses, pulsedurations are typically measured by the full width at half maximum(FWHM) technique.

As used herein, the recitation of a numerical range for a variable isintended to convey that the embodiments may be practiced using any ofthe values within that range, including the bounds of the range. Thus,for a variable which is inherently discrete, the variable can be equalto any integer value within the numerical range, including theend-points of the range. Similarly, for a variable which is inherentlycontinuous, the variable can be equal to any real value within thenumerical range, including the end-points of the range. As an example,and without limitation, a variable which is described as having valuesbetween 0 and 2 can take the values 0, 1 or 2 if the variable isinherently discrete, and can take the values 0.0, 0.1, 0.01, 0.001, orany other real values ≥0 and ≤2 if the variable is inherentlycontinuous. Finally, the variable can take multiple values in the range,including any sub-range of values within the cited range.

As used herein, unless specifically indicated otherwise, the word “or”is used in the inclusive sense of “and/or” and not the exclusive senseof “either/or.”

In accordance with various aspects of the applicants' teachings, thesystems and methods described herein can be effective to deliverpicosecond pulses of laser radiation for the treatment ofnaturally-occurring and artificial skin pigmentations utilizingwavelengths that match the pigmentations' absorption spectrum. Thepicosecond, high power pulses disclosed herein can be particularlyeffective in removing these previously-difficult to treat skinpigmentations, in part, because the pulses induce photomechanical shockwaves at the target sites that cause greater disruption and betterclearance of pigment particles. By way of example, the methods andsystems disclosed herein can improve the clearing of red and orangetattoos with a reduced number of treatments by delivering picosecondlaser pulses having a wavelength between 400 and 550 nm, where thesepigments exhibit much higher absorption coefficients. Moreover,applicants have discovered that various embodiments of thewavelength-shifting resonators described herein can surprisinglygenerate particularly efficacious pulses exhibiting picosecondpulsewidths shorter than the input pumping pulses, with low energylosses and/or minimal pulse-shaping (e.g., without use of a modelocker,Q-switch, pulse picker or any similar device of active or passive type).

The present disclosure relates to laser systems having sub-nanosecondpulsing (e.g., picosecond pulsing). Exemplary systems are described inour U.S. Pat. Nos. 7,929,579 and 7,586,957, both incorporated herein byreference. These patents disclose picosecond laser apparatuses andmethods for their operation and use. Herein we describe certainimprovements to such systems.

With reference now to FIG. 1 , an exemplary system 100 for thegeneration and delivery of picosecond-pulsed treatment radiation isschematically depicted. As shown in FIG. 1 , the system generallyincludes a pump radiation source 110 for generating picosecond pulses ata first wavelength, a wavelength-shifting resonator 120 for receivingthe picosecond pulses generated by the pump radiation source andemitting radiation at a second wavelength in response thereto, and atreatment beam delivery system 130 for delivering a pulsed treatmentbeam to the patient's skin.

The pump radiation source 110 generally generates one or more pulses ata first wavelength to be transmitted to the wavelength-shiftingresonator 120, and can have a variety of configurations. For example,the pulses generated by the pump radiation source 110 can have a varietyof wavelengths, pulse durations, and energies. In some aspects, as willbe discussed in detail below, the pump radiation source 110 can beselected to emit substantially monochromatic optical radiation having awavelength that can be efficiently absorbed by the wavelength-shiftingresonator 120 in a minimum number of passes through the gain medium.Additionally, it will be appreciated by a person skilled in the art inlight of the present teachings that the pump radiation source 110 can beoperated so as to generate pulses at various energies, depending forexample, on the amount of energy required to stimulate emission by thewavelength-shifting resonator 120 and the amount of energy required toperform a particular treatment in light of the efficiency of the system100 as a whole.

In various aspects, the pump radiation source 110 can be configured togenerate picosecond pulses of optical radiation. That is, the pumpradiation source can generate pulsed radiation exhibiting a pulseduration less than about 1000 picoseconds (e.g., within a range of about500 picoseconds to about 800 picoseconds). In an exemplary embodiment,the pump radiation source 110 for generating the pump pulse at a firstwavelength can include a resonator (or laser cavity containing a lasingmedium), an electro-optical device (e.g., a Pockels cell), and apolarizer (e.g., a thin-film polarizer), as described for example withreference to FIG. 2 of U.S. Pat. No. 7,586,957, issued on Sep. 8, 2009and entitled “Picosecond Laser Apparatus and Methods for Its Operationand Use.” the contents of which are hereby incorporated by reference inits entirety.

In an exemplary embodiment, the lasing or gain medium of the pumpradiation source 110 can be pumped by any conventional pumping devicesuch as an optical pumping device (e.g., a flash lamp) or an electricalor injection pumping device. In an exemplary embodiment, the pumpradiation source 110 comprises a solid state lasing medium and anoptical pumping device. Exemplary solid state lasers include analexandrite or a titanium doped sapphire (TIS) crystal, Nd:YAG lasers,Nd:YAP, Nd:YAlO₃ lasers, Nd:YAF lasers, and other rare earth andtransition metal ion dopants (e.g., erbium, chromium, and titanium) andother crystal and glass media hosts (e.g., vanadate crystals such asYVO₄, fluoride glasses such as ZBLN, silica glasses, and other mineralssuch as ruby). At opposite ends of the optical axis of the resonator canbe first and second mirrors having substantially complete reflectivityand/or being substantially totally reflective such that a laser pulsetraveling from the lasing medium towards second mirror will first passthrough the polarizer, then the Pockels cell, reflect at second mirror,traverse Pockels cell a second time, and finally pass through polarizera second time before returning to the gain medium. The terms“substantially complete reflectivity” and/or “substantially totallyreflective” are used to indicate that the mirrors completely reflectincident laser radiation of the type normally present during operationof the resonator, or reflect at least 90%, preferably at least 95%, andmore preferably at least 99% of incident radiation.

Depending upon the bias voltage applied to the Pockels cell, someportion (or rejected fraction) of the energy in the pulse will berejected at the polarizer and exit the resonator along an output path tobe transmitted to the wavelength-shifting resonator 120. Once the laserenergy, oscillating in the resonator of the pump radiation source 110under amplification conditions, has reached a desired or maximumamplitude, it can thereafter be extracted for transmission to thewavelength-shifting resonator 120 by changing the bias voltage to thePockels cell such that the effective reflectivity of the second mirroris selected to output laser radiation having the desired pulse durationand energy output.

The wavelength-shifting resonator 120 can also have a variety ofconfigurations in accordance with the applicant's present teachings, butis generally configured to receive the pulses generated by the pumpradiation source 110 and emit radiation at a second wavelength inresponse thereto. In an exemplary embodiment, the wavelength-shiftingresonator 120 comprises a lasing medium and a resonant cavity extendingbetween an input end and an output end, wherein the lasing mediumabsorbs the pulses of optical energy received from the pump radiationsource 110 and, through a process of stimulated emission, emits one ormore pulses of optical laser radiation exhibiting a second wavelength.As will be appreciated by a person skilled in the art in light of thepresent teachings, the lasing medium of the wavelength-shiftingresonator can comprise a neodymium-doped crystal, including by way ofnon-limiting example solid state crystals of neodymium-dopedyttrium-aluminum garnet (Nd:YAG), neodymium-doped pervoskite (Nd:YAP orNd:YAlO₃), neodymium-doped yttrium-lithium-fluoride (Nd:YAF), andneodymium-doped vanadate (Nd:YVO₄ crystals. It will also be appreciatedthat other rare earth transition metal dopants (and in combination withother crystals and glass media hosts) can be used as the lasing mediumin the wavelength-shifting resonator. Moreover, it will be appreciatedthat the solid state laser medium can be doped with variousconcentrations of the dopant so as to increase the absorption of thepump pulse within the lasing medium. By way of example, in some aspectsthe lasing medium can comprise between about 1 and about 3 percentneodymium.

The lasing medium of the wavelength-shifting resonator 120 can also havea variety of shapes (e.g., rods, slabs, cubes) but is generally longenough along the optical axis such that the lasing medium absorbs asubstantial portion (e.g., most, greater than 80%, greater than 90%) ofthe pump pulse in two passes through the crystal. As such, it will beappreciated by a person skilled in the art that the wavelength of thepump pulse generated by the pump radiation source 110 and the absorptionspectrum of the lasing medium of the resonator 120 can be matched toimprove absorption. However, whereas prior art techniques tend to focuson maximizing absorption of the pump pulse by increasing crystal length,the resonator cavities disclosed can instead utilize a short crystallength such that the roundtrip time of optical radiation in the resonantcavity (i.e., t_(roundtrip)=2 L_(resonator)/C, where L_(resonator) isthe optical path length of the resonator (the optical path length canaccount for differences due to the photons traveling through the lasingmedium and/or the air in other parts of the path) and c is the speed oflight) in some embodiments the optical path length is substantially lessthan the pulse duration of the input pulse (i.e., less than the pulseduration of the pulses generated by the pump radiation source 110). Forexample, in some aspects, the roundtrip time can be less than 5 timesshorter than the duration of the picosecond pump pulses input into theresonant cavity (e.g., less than 10 times shorter). Without being boundby any particular theory, it is believed that by shortening the resonantcavity, the output pulse extracted from the resonant cavity can have anultra-short duration without the need for additional pulse-shaping(e.g., without use of a modelocker, Q-switch, pulse picker or anysimilar device of active or passive type). For example, the pulsesgenerated by the wavelength-shifting resonator can have a pulse durationless than 1000 picoseconds (e.g., about 500 picoseconds, about 750picoseconds).

After the picosecond laser pulses are extracted from thewavelength-shifting resonator 120, they can be transmitted directly tothe treatment beam delivery system 130 for application to the patient'sskin, for example, or they can be further processed through one or moreoptional optical elements shown in phantom, such as an amplifier 140,frequency doubling waveguide 150, and/or filter (not shown). As will beappreciated by a person skilled in the art, any number of knowndownstream optical (e.g., lenses) electro-optical and/or acousto-opticelements modified in accordance with the present teachings can be usedto focus, shape, and/or alter (e.g., amplify) the pulsed beam forultimate delivery to the patient's skin to ensure a sufficient laseroutput, while nonetheless maintaining the ultrashort pulse durationgenerated in the wavelength-shifting resonator 120.

With reference now to FIG. 2 , an exemplary system 200 is depicted thatincludes a wavelength-shifting resonator 220 as described for example inFIG. 1 . As shown in FIG. 2 , however, the system 200 can also be usedto generate and selectively apply multiple wavelengths of picosecondpulses depending, for example, on the absorption spectrum of the targetpigment or tissue. As shown in FIG. 2 , the exemplary system generallyincludes a pump radiation source 210 for generating picosecond pulses ata first wavelength (e.g., an alexandrite source emitting 755 nm pulseshaving a duration less than 1000 picoseconds), at least one opticalelement (M1 and/or M2) configured to selectively divert the picosecondpulses at the first wavelength to a wavelength-shifting resonator 220(e.g., a 1064 nm oscillator configured to receive the pump pulses andgenerate 1064 nm picosecond pulses of radiation in response thereto), atleast one optical element (M3 and/or M4) and a treatment beam deliverysystem 230 that can deliver the picosecond pulses of one or morewavelengths to the treatment target. As shown in phantom, and discussedotherwise herein, the system 200 can additionally include, for example,an amplifier 240 and a second harmonic generator 250 (e.g., a lithiumtriborate (LBO) or potassium trianyl phosphate (KTP) frequency doublingcrystal).

As discussed above, the wavelength-shifting resonator 220 can comprise arare earth doped laser gain crystal. In some aspects, rare earth dopedlaser crystals that generate a polarized laser beam like Nd:YVO₄ can beused to eliminate the need for an additional polarizing element.Crystals like Nd:YAG or Nd doped glasses can be used with an additionalpolarizing element in the resonator. In the exemplary embodiment, theinput side of the Nd:YVO₄ crystal can be AR coated for the alexandritewavelength and HR coated for 1064 nm, while the output side of thecrystal can be IR coated for the alexandrite wavelength and can exhibitapproximately 20 to 70% reflectivity at 1064 nm. In an exemplaryembodiment, the Nd:YVO₄ crystal length can be selected such that itabsorbs most (greater than 90%) of the alexandrite laser pulse in thetwo passes through the crystal. For example, with neodymium doping inthe range 1 to 3%, the Nd:YVO₄ crystal can be chosen to be around 3 mmlong (with no other optical elements in the resonator, the resonatorlength is substantially equal to the crystal length of 3 mm). That meansthe resonator round-trip time is around 39 ps—substantially less thanthe pulse duration of the alexandrite pumping pulse (around 500 to 800ps). The 1064 nm pulse generated in the very short round trip timeNd:YVO₄ resonator may be slightly longer than the pumping alexandritepulse and shorter than 1000 ps. The quantum defect will account for a30% pulse energy loss and another 15% of the energy is likely to be lostdue to coatings, crystal and geometry imperfection, for an overallenergy conversion efficiency of around 50 to 60% such that 100 mJ pulseenergy can be produced at 1064 nm, by way of non-limiting example. Inthe Second Harmonic Generator 250 the second harmonic conversion in thefrequency-doubling crystal is around 50%, such that a 50 mJ pulse energycan therefore be produced at 532 nm. Given the high absorption at 532 nmof red and/or orange tattoo pigments, a 50 mJ, 532 nm pulse with a pulseduration less than 1000 picoseconds can be effective at disrupting, andeventually clearing, red and/or orange tattoo granules.

Though the above described example utilized an Nd:YVO₄ crystal in thewavelength-shifting resonator 220 (and without the need for a polarizingelement), Nd:YAG crystals or other Nd-doped glasses can alternatively beused as the short resonator to generate the picosecond pulses inresponse to stimulation from the pump radiation source. In suchembodiments, a polarizing element as known in the art can be utilizedexternal to the wavelength-shifting resonator or can be embeddedtherein. As shown in FIG. 3 , for example, a short Nd:YAG resonator 322can consist of two identically shaped crystals 322 a,b with one face 324cut at an angle that is AR coated for the alexandrite wavelength andpolarized-coated for the stimulated emission wavelength (e.g., 1064 nmand high p transmission). The flat faces 326 of the two Nd:YAG crystalscan have different coatings-one is AR coated at 755 nm and HR coated at1064 nm and the other is HR coated for 755 nm and has an output couplerreflectivity around 50 to 80% for 1064 nm. The higher output couplerreflectivity for the Nd:YAG crystal compared to the Nd:YVO₄ crystal isdue to the lower gain cross-section in Nd:YAG.

With reference again to FIG. 2 , it will be appreciated in light of thepresent teachings that utilizing a wavelength-shifting resonator 220 togenerate picosecond pulses depends on the pulse duration of the pumpingpulse (e.g., shorter pumping pulses will lead to shorter generatedpulses at 1064 nm) and the roundtrip time determined by the length ofthe resonator cavity (e.g., shorter crystals lead to shorter roundtriptime, however the crystal has to be sufficiently long to absorb greaterthan 90% of the alexandrite energy). For example, an 8 mm long Nd:YAGresonator would have a 97 ps round trip time. Though such a roundtriptime is longer than the roundtrip time that can be achieved with aNd:YVO₄, resonator, it remains much shorter than the pumping Alexandritelaser pulse duration. It will be appreciated by a person skilled in theart in light of the present teaching that one possible way to shortenthe crystal length is to tune the alexandrite laser in the range 750 to760 nm for maximum absorption in the Nd doped crystal and use theminimum possible crystal length. In addition, by tuning the alexandritelaser in the range of 750 to 757 nm allows for the alexandritewavelength to be set to avoid the excited state absorption bands in theNd ion as described by Kliewer and Powell, IEEE Journal of QuantumElectronics vol. 25, page 1850-1854 (1989).

With reference again to FIG. 2 , the laser beam emitted by the pumpradiation source 210 (e.g., an alexandrite laser source generatingpulses at around 755 nm and 200 mJ/pulse, with a pulse duration shorterthan 800 ps) can be reflected on 100% reflectors M1 and M2 to serve asthe pump beam for the wavelength-shifting resonator 220 (e.g., anNd:YVO₄ or Nd:YAG short round trip time 1064 nm oscillator), therebystimulating the oscillator 220 to produce up to around 100 mJ pulseenergy at 1064 nm at less than 1000 ps pulse duration. The output firmthe 1064 nm oscillator 220 can be reflected on the 100% reflectors M3and M4 and can be coupled into the treatment beam delivery system 230.

Alternatively, the output from the 1064 nm oscillator 220 can beamplified in the 1064 nm amplifier 240 to a pulse energy between 200 and900 mJ, for example, while maintaining the less than 1000 ps pulseduration and then reflected on the 100% reflectors M3 and M4 and coupledinto the treatment beam delivery system.

Alternatively or additionally, the output from the 1064 nm oscillator220 or the output from the 1064 nm amplifier 240 can be converted tosecond harmonic 532 nm radiation in the Second Harmonic Generator 250.For a typical 50% conversion efficiency in the Second Harmonic Generator250, the 532 nm pulse output can have a pulse energy around 50 mJ whenthere is no 1064 nm amplifier, or between 100 to 500 mJ when the 1064 nmpulse is amplified in the 1064 nm amplifier 240 before it reaches theSecond Harmonic Generator 250. In both cases, the 532 nm pulse will havea pulse duration of around 750 ps or less due to the pulse shorteningeffect of the second harmonic conversion process. After being frequencydoubled, the 532 nm pulse can propagate in parallel with the 1064 nmpumping pulse. By choosing mirrors M3 and M4 to be 100% reflectors orsubstantially totally reflective reflectors on both the 1064 nm and 532nm wavelengths, a combined wavelength treatment can be delivered to thetarget through the treatment beam delivery system.

Alternatively, in some embodiments, mirrors M3 and M4 can be chosen tobe 100% reflectors at 532 nm and 100% transmitters at 1064 nm so as todeliver a single wavelength 532 nm treatment through the treatment beamdelivery system. Moreover, by allowing the mirrors (M1, M2) toselectively transmit or deflect the 755 nm alexandrite pulse, forexample, by translating the mirrors into and out of the pulse beam, thesystem 200 can be designed to transmit all three treatment wavelengths755, 1064 and 532 nm. That is, when mirrors M1 and M4 are moved out ofthe beam path of the pump radiation source 210, the pulsed pump beam of755 nm is coupled directly to the treatment beam delivery system to beused for patient treatments.

Example

An example plot of the output pulse shape of a short wavelength-shiftingNd:YAG resonator with a 70% output coupler is shown in FIG. 4 , asmeasured at position (B) of FIG. 1 . The Nd:YAG crystal was doped to 1.3at. % (30% higher than the standard 1 at. % doping) to allow for ashorter resonator—6.2 mm in length, shorter roundtrip time, and ashorter output pulse duration. The Nd:YAG oscillator was pumped by anAlexandrite laser with 200 mJ per pulse, 680 ps pulse duration, and a4.4 mm spot (as measured at position (A) of FIG. 1 ). As shown in FIG. 4, the output of the wavelength-shifting Nd:YAG resonator at 1064 nm was65 mJ per pulse, 750 ps mean pulse duration. The roundtrip time in theNd:YAG resonator was about 76 picoseconds, substantially shorter thanthe 680 ps Alexandrite input pulse.

With reference now to FIG. 5 , the output pulse shape of a shortresonator Nd:YVO₄ laser having a length of 3 mm with a 50% outputcoupler is depicted, as measured at position (B) of FIG. 1 . The Nd:YVO₄oscillator was pumped by the output of an Alexandrite laser delivering200 mJ per pulse, 720 ps pulse duration focused to a 6.3 mm spot (asmeasured at position (A) of FIG. 1 ). The pump spot was apertured downto 3.6 mm diameter. As shown in FIG. 5 , the output wavelength-shiftingNd:YVO₄ resonator at 1064 nm was 34 mJ per pulse, 500 ps mean pulseduration. It is surprising that the output pulse duration of the shortpulse Nd:YVO₄ laser resonator is shorter than the pulse duration of thepumping Alexandrite pulse—500 ps relative to 720 ps, especiallyconsidering that the shorter output pulse duration is achieved withoutany extra elements in the laser resonator aimed at pulse shaping (e.g.,in the resonator there is no modelocker. Q-switch, pulse picker or anysimilar device of active or passive type). The short Nd:YVO₄ resonatoris also remarkably and surprisingly efficient. That is, with 33% of theAlexandrite energy being transmitted through the aperture (i.e., 66 mJ),the 34 mJ Nd:YVO₄ resonator output is 51% of the pump energy transmittedthereto. The roundtrip time in the Nd:YVO₄ resonator was about 39 ps,substantially shorter than the 720 ps Alexandrite input pulse.

An example plot of the output pulse shape of a short resonator Nd:YAGlaser with a 70% output coupler is shown on FIG. 6 . The Nd:YAGoscillator was pumped with 200 mJ per pulse, 860 ps pulse duration, 4 mmspot. The oscillator output at 1064 nm was 96 mJ per pulse, 1030 pspulse duration. FIG. 6 shows that it is possible to generate and have anoutput that has a longer pulse duration 1030 ps than the pulse durationof the pumping pulse, 860 ps.

The short pulse output from the short roundtrip time oscillator (e.g.,resonator) can be amplified to increase the pulse energy while keepingthe pulse duration shorter than 1000 ps as described previously. Whenthe oscillator and amplifier material are the same, for example Nd:YAGor Nd:YVO₄, the oscillator output wavelength can be matched to theamplifier gain profile to enable maximum energy extraction from theamplifier.

In one embodiment, the oscillator and amplifier materials are differentfrom one another, optionally, it is advantageous for the oscillator tobe made from different materials than the amplifier. For example aNd:YVO₁ oscillator can be designed with a shorter roundtrip time vs aNd:YAG oscillator, and a shorter output pulse duration will be producedby the Nd:YVO₄ oscillator when pumped with a short pulse Alexandritelaser, as compared to a Nd:YAG oscillator as discussed previously.Amplifying the Nd:YVO₄ oscillator output in a Nd:YVO₄ amplifier isrelatively difficult because of the shorter fluorescence lifetime ofNd:YVO₄ is 100 ps versus the 230 ps fluorescence lifetime of the Nd:YAG.Amplifying the Nd:YVO₄ oscillator output in a Nd:YAG amplifier ispossible, but sub-optimal because of the wavelength mismatch of the twodifferent materials. According to Koechner “Solid-State LaserEngineering”, 5^(th) Ed., the laser wavelength of Nd:YVO₄ is 1064.3 nm,while the Nd:YAG peak gain wavelength is 1064.1 nm.

More detailed data for the laser output wavelength of a Nd:YVO₄oscillator is published by Mingxin et al. “Performance of a Nd:YVO₄microchip laser with continuous-wave pumping at wavelengths between 741and 825 nm”, Appl. Opt. v. 32, p. 2085, where the laser outputwavelength of a Nd:YVO₄ microchip laser is shown to vary when theoscillator temperature is varied such that the laser output is 1063.9 nmwhen the oscillator temperature is about 0° C., and the laser output is1064.5 nm when the oscillator temperature is about 100° C. An optimizedlaser system consisting of a Nd:YVO₄ oscillator and a Nd:YAG amplifiercan be envisioned where the temperature of the oscillator and/or theamplifier is controlled and/or adjusted such that and the peakwavelength can be varied. In one embodiment, one controls thetemperature of the Nd:YVO₄ so that it is well amplified in theamplifier. In one embodiment, the temperature of the oscillator and/orthe amplifier is controlled so that one can provide a maximum energyoutput pulse with a minimal pulse duration. The range of temperatureadjustment can be between about 0° C. and about 100° C., between about20° C., and about 80° C., or between about 30° C., and about 70° C.

In addition to temperature control, other possible approaches tocontrolling and/or varying the peak wavelength can include externalpressure applied to the laser material and doping the laser materialwith trace amounts of elements that would alter, for example, thecrystal lattice stress. The approaches to varying peak wavelength suchas oscillator and/or amplifier temperature control, pressure applied tothe laser material, and doping the laser material can be employed aloneor in combination.

Gain Uniformity

Gain uniformity in the lasing medium of a laser (e.g., in a solid statealexandrite lasing medium) has a direct effect on the uniformity of theoutput beam. In the case of a multi-mode, mode locked laser, asdiscussed previously herein (e.g., at FIGS. 1 and 2 ), where the beamenergy propagates through the gain medium multiple times, a differencein gain uniformity of only a few percent can cause undesired modes withhigh peak fluences to develop. Gain uniformity is important because inthe early stages of laser profile generation differences in gainuniformity in the lasing medium (e.g., a rod) have an exponential buildup. Relatively small differences in the lasing medium gain profile (thisis the pump profile) become exacerbated. To optimize the energyextracted from the resonator, a relatively even fluence is mostdesirable, for example, a round beam of even fluence is preferred. It isdesirable to obtain a more uniform fluorescence profile such that thecenter of the lasing medium, for example, a crystal rod and its edgeshave substantially the same amount of fluorescence (e.g., a relativelyeven fluorescence).

In order to generate light via a light source (e.g., a pumped radiationsource such as a flash lamp) the light couples into a lasing medium(e.g., a crystal laser rod) and that coupling can be done via areflecting enclosure. The reflector can be diffuse (e.g., scattered) orspecular (e.g., like a silvered surface that is mirror-like and notscattered). The lasing medium (e.g., crystal rod) absorbs the lightcoupled into it from the flash lamp. An absorption profile develops inlasing medium (e.g., the crystal rod). The function of the lasing mediumis to absorb the light from the flash lamp and then to re-emit the lightat changed wavelength (e.g., a longer wavelength). Where the lasingmedium is a crystal rod if the middle of the rod absorbed the most lightthe middle of the rod would appear to be the brightest in that emittedwavelength—i.e., to emit the most changed wavelength. The phenomenon ofthe rod center being brighter than the rod edges is referred to as“fluorescence non-uniformity” this can generally occur for any laserwhere a flash lamp is coupled to a crystal (e.g., a crystal rod).

Turning now to FIG. 7 , a cross section of a traditional dual-flash lampdiffuse pump chamber is depicted. Two flash lamps 713, each encased inglass coolant tubes 715, are arranged in parallel on both sides of acentral lasing medium 711 (e.g., an alexandrite crystal rod lasingmedium). The two flash lamps 713 and the lasing medium crystal 711 areall encased within a diffusing material 717 as shown in FIG. 7 . Anydiffusing material which would survive the high intensity light from theflash lamps 713 is suitable. Suitable modifications can includesandblasting a texture on the flash lamps 713 for example, on thecoolant tubes 715 that encase the flash lamps 713, or in an area betweenthe flash lamps 713 and the crystal lasing rod 711, shown in FIG. 7and/or providing a coated a strip of aluminum with a white diffusingcoating for example on one or more of the flash lamps 713 (e.g., on thecoolant tube(s) 715). Some white diffusing coating examples includepotassium sulfate, aluminum oxide, compressed PTFE and fumed silica.

Many factors can contribute to non-uniform gain distribution within thelasing medium. Lasing medium crystals may have different absorptioncoefficients at different wavelength(s) and/or along different axis ofthe crystal. This can be further imbalanced by the unequal outputspectrum of the flash lamp pump source and how it matches the absorptionspectrum of the lasing medium 711 (e.g., the active lasing medium).There is also the magnitude of the quantum defect within the flash lamppump bands. It is desirable to improve gain uniformity on any materialwhich lases, and the choice of lasing media is considered to be withinthe skill of an ordinary practitioner in view of the teachings providedherein.

The pump chamber geometry can also contribute to non-uniform gain bycoupling more light into the crystal along one direction. In the case ofan alexandrite crystal in a diffuse pump chamber, an increase in gainwas observed in the direction of the flash lamps.

FIG. 8 depicts an image generated by the pump chamber geometry of FIG. 7, and captured by aligning a camera to the axis of the lasing medium 811(e.g., the alexandrite crystal laser rod) and imaging the fluorescenceof the pumped lasing medium 811 (e.g., the alexandrite crystal laserrod). The end face 814 of the lasing medium 811 is depicted as having asubstantially circular boundary. The areas of the laser rod end face 814that are most proximate to the flash lamps 813 exhibit high gain regions812.

FIG. 9 depicts a graph showing profiles of the lasing medium 811described in FIGS. 7 and 8 with the profiles taken along the horizontalaxis 818B and along the vertical axis 818A. As can be seen by the graphin FIG. 9 , the gain at the edge regions 812 of the end face 814 of thelasing medium 811 (in FIG. 8 ) in the horizontal direction 818B may beabout 5 to 10 percent higher than the gain in the vertical direction818A. These edge region 812 peaks correspond to the high gain regionsdepicted in the image of FIG. 8 . This uneven gain distribution isproblematic in that it leads to failure of the laser system due, forexample, to uneven heating of the lasing medium that results in systembreakdown and unacceptable down time and repair times. Further, wherethere is substantially uniform beam gain one can increase the systempower output with less system failure than in the system where the gainis not uniform.

Accordingly, in order to improve system reliability, it is desirable tolessen and/or eliminate these gain peaks such that gain is substantiallyuniform across all axis of the lasing medium (e.g., that the gain issubstantially uniform along both the horizontal axis 818B and along thevertical axis 818A of the lasing medium).

Embodiments of the present disclosure that improve gain uniformityinclude an optical system comprising a pump chamber with one or moreelements that enable a substantially uniform gain across the lasingmedium, for example, diffusing element(s) disposed between a flash lampand a crystal. Elements that enable a substantially uniform gain acrossthe lasing medium can include, for example, light shaping elements forexample deflectors that lead to diffusion, scattering, refraction,and/or reflection or elements that provide absorption. In oneembodiment, the element that enables a substantially uniform gain acrossthe lasing medium is a diffusing element that acts to scatter a portionof the light coupling into the crystal and to increase the diffuseillumination of the rod, thereby avoiding non-uniform high-gain regionsand achieving a circular symmetry to the gain region within the crystalrod.

Referring to FIGS. 10 and 11 , relatively uniform fluorescence can beachieved via elements that enable a substantially uniform gain acrossthe lasing medium. Suitable elements include diffusing elements 1019.The stored photons from the rod fluoresce and enter into the cavity ofthe resonator formed by two or more mirrors. The photons travel betweenat least two mirrors that are along the relatively long axis of the gainmedium and the photons build up energy through multiple trips betweenthe opposing mirrors, which are substantially totally reflective. It isduring this buildup of energy that the impact of the contrast between anon-uniform fluorescence and a relatively uniform fluorescence can bebest understood when considering the laser energy profile that isemitted. A non-uniform fluorescence results in non-uniform energyemission from the laser, which is problematic due to the wear it causeson, for example, the optical components of the laser. For example,coatings present on the Pockels cells can be deteriorated by thenon-uniform energy emission. A more uniform fluorescence results in amore uniform energy emission from the laser, which is desirableincluding due to the resulting increase in optical component longevity.By using an element that improves gain uniformity, such as a diffuser1019 (e.g., the baffle and/or an absorber) obtaining more uniform gainand thereby more uniform fluorescence is favored, but at the expense ofpumping efficiency, which is sacrificed due the presence of the element1019.

Referring still to FIGS. 10 and 11 , according to one embodiment of thedisclosure, a lasing medium 1011 (e.g., a crystal rod) was placedbetween each flash lamp 1013 and a diffusing element 1019 (e.g., abaffle and/or an absorber) was placed in between the flash lamp 1013 andthe lasing medium 1011 to scatter a portion of the light couplingdirectly into the lasing medium 1011 and to increase the diffuseillumination of the crystal rod lasing medium 1011. According to oneembodiment, when a suitably-sized diffusing element 1019 was placed inthe chamber, the lasing medium 1011 (e.g., a crystal rod) achievedsubstantially uniform gain (e.g., substantially circular symmetry). Inone embodiment, the diffusing element 1019 is a 0.063 inch diameteralumina rod that was placed equidistant between the flash lamp 1013 andthe crystal rod lasing medium 1011. Suitable diffusing element 1019diameters can be about the same diameter as the lasing medium (e.g.,about 0.375 inches) to as small a diameter as can be structurally sound(e.g., about 0.03 inches).

FIG. 10 depicts an image of the fluorescence using such animplementation. The fluorescence resulting from this chamberconfiguration shows the gain is more evenly distributed in a circularlysymmetric fashion and the high-gain regions seen in FIG. 8 (whendiffusing elements are absent) are eliminated.

The graph of FIG. 11 shows that the horizontal and vertical beamprofiles have a closer agreement between the gain in the two axes (e.g.,the vertical axis 1018A and horizontal axis 1018B show a substantiallysimilar gain distribution) of the beam are in close agreement. Thenormalized gain distribution in the chamber having the diffusing elementshows that the edge region 1012 of FIG. 10 lacks the peaks seen in FIG.9 that were a result when there was no diffusing element in place.

In some embodiments, the choice of gain uniformity element material(e.g., a diffuser, absorber, deflector, baffle, scattering element,refractor, and/or reflector) and in the case of a material in the shapeof a rod the selected diameter of the gain uniformity material can beadjusted to improve the beam uniformity of the system. The gainuniformity element (e.g., the diffusing element) need not sit betweenthe flash lamp and the laser rod, rather the diffusing element can be agrating that is etched on the surface of one or more of the flash lampor the laser rod.

The effect of balancing and/or improving gain uniformity on the beamprofile of a mode locked laser by altering pump chamber geometry, e.g.,by adding one or more diffusing element, is dramatic. The image depictedin FIG. 12 shows the beam produced by the unmodified chamber describedin connection with FIG. 8 . The high peak fluence produced at the sidesof the beam in FIG. 12 are beyond the damage threshold of the opticscontained in the laser resonator. In comparison, the beam profile shownin FIG. 13 , is produced by an embodiment that includes one or morediffusing elements (e.g., a baffled chamber with an alumina rod) likethat described in connection to FIG. 10 and the beam profile is producedby the modified chamber is more circular, indicating the energy from thebeam is spread over a greater area. As a result, the peak fluence of thebeam generated with the embodiment of the pump chamber modified toinclude at least one diffusing element was greatly reduced and overallsystem power may be increased without damaging the optics in theresonator. As a result, the life and/or reliability of the laser systemis improved due to the presence of the at least one gain uniformityimprovement elements (e.g., a baffle).

Non-Spherical Lenses Lessen Free Space Propagation Mode Effect

Picopulse laser treatment energy relies on laser intensity, which is thesquare of the sum of the lasers electric fields. When free spacepropagation modes couple together the laser output intensity profile cantend toward non-uniformity. Free space propagation modes can include oneor more of Hermite profiles, Leguerre profiles and Ince Gaussianprofiles.

For the multi transverse mode laser it is beneficial to have sufficienttransverse modes present such that the beam profile is filled in(substantially even). This ensures the peak fluence will be as close aspossible to the average fluence. The ideal situation is the where thebeam profile has a “top hat” beam profile, which looks like a top hat inprofile e.g., referring to the representation of the normalized gaindistribution shown in FIG. 1 in an idealized situation the two gainregions 1012 connect with a straight line and the sloping sides are muchsteeper. A low peak fluence will prevent laser damage to opticalcoatings and thus prolong the life of the laser.

The picopulse resonator can produce many multimode Hermite profileelectric fields and can produce unwanted combinations of multimodeHermites. In order meet the desired laser treatment energy levels.Hermite profiles can result in high intensity profiles. These highintensity profiles can damage the optics of the resonator leading toreduced lifetime issues.

It is desirable to lessen the impact of free space propagation modesincluding Hermites in the beam output profile. Introducing a lenselement that provides astigmatism can act to decouple free spacepropagation modes thereby obviating or lessening their impact on thebeam profile. Lens elements that can lessen the impact of free spacepropagation modes (e.g., Hermites) could be for example, cylinder lens,angled spherical lens, anamorphic prisms, etc.

Unlike a spherical lens, which is cut from a sphere, a non-sphericallens (e.g., an astigmatic lens such as a cylindrical lens) can be cutfrom a rod. Specifically, a non-spherical lens can be cut along the longaxis of a rod such that its end face looks like the letter “D”. Thenon-spherical lens provides only one axis of curvature in contrast to aspherical lens which provides two axis of curvature. When light travelsthrough the curved axis the light is deviated (e.g., focused ordefocused) by the curvature of the lens such that the light is differentin the x-plane versus the y-plane. Light that travels across the otheraxis does not get focused or defocused—it sees no deviation.

Alternatively, a spherical lens may be angled such that light impingeson the spherical lens at an angle that provides the effect of acylindrical lens such that the angled spherical lens output of light isdifferent in the x-plane versus the y-plane. These are just a few ofmany ways to produce an astigmatic lens effect. Other methods or meansof utilizing lenses or prisms to produce an astigmatic effect are knownto those of skill in the relevant art.

There is a phenomenon in multimode lasers by which multiple Hermiteprofiles can build up within a resonator and interfere with each otherto cancel portions of one another out and thereby create hotspots thatgive an unacceptable laser beam intensity profile. By controlling and/ormanaging the mix of Hermites their interference in the laser output canbe limited. The mix of Hermites can be limited by utilizing differentastigmatism for the x and y axis' in the resonator. In this way, theastigmatic element prevents multiple Hermite profiles from interferingwith one another to produce a bad profile. Rather, each individualHermite profile exits the laser individually. In this way, theastigmatic lens element avoids Hermite's canceling portions of oneanother out that results in undesirable hot spots in the laser beamprofile that can cause wear on the optics of the system. As discussedpreviously, it is important to provide a beam output that shows arelatively even energy distribution (e.g., beam uniformity). Theastigmatic effect element can aid in beam uniformity, because it avoidscoupling of free space propagation modes that result in undesirable hotspots.

An example of an unwanted two electric field combination with aresultant laser intensity combination is shown FIG. 14A. In FIG. 14A,the majority of the beam energy is contained in two distinct regions1412 within the profile. This is an unwanted electric field, which is aresult of the combination of two individual propagating Hermite fieldsthat remain in phase, i.e. each field is in step with the other.

By introducing astigmatism into the picopulse resonator the undesirablephase relationship of the propagating Hermite electric fields is brokenalong the astigmatic axes (physics Gouy phase effect). Using the sametwo Hermites of the previous combination example shown in FIG. 14A, butnow showing the effect of phase mismatch created by the astigmaticelement (e.g., astigmatic lens) on intensity is FIG. 14B. The FIG. 14Bprofile has a better fill of energy or distribution of energy in thatall four corners of the beam profile are illuminate, which is much lesslikely to damage the optics compared to the beam profile in FIG. 14Awhere energy is concentrated into two of the four corners of the beamprofile.

The picopulse laser transverse mode profile is improved when astigmatismis introduced into the resonator. The astigmatism essentially providestwo resonator configurations, each with a preferred set of modes. In oneembodiment, astigmatism was introduced by a weak cylindrical lens «0.5Dioptres. The astigmatic generating element could be placed anywherewithin the resonator path. The cylinder lens worked well when its axiswas perpendicular or parallel to the plane polarized light in thepicopulse laser.

There are many approaches to introducing an astigmatic element to theresonator, for example, the goal of different net curvature can beachieved within a resonator by, for example, positioning a sphericallens or spherical lenses such that one or more spherical lens is tiltedrelative to the optical axis, thereby providing one or more astigmaticelement(s). Alternatively, the beam can be expanded in a singledirection (e.g., anamorphic expansion) prior to a lens or a sphericalmirror.

Another method of free space propagation mode control is to place anobscuration (e.g., a wire) at the electric field zero crossings of awanted mode. The obscuring element (e.g., for a Hermite a line, foraLeGuerre a radial obscuration) can be produced in a substrate or in theanti-reflection coating on a substrate. The obscuration element preventsunwanted free space propagation modes from lasing and effectivelyfilters them out of the distribution of energy lased from the system.Preferably, obscuring elements have thin lines (e.g., lines that are <50um thick), which can be produced, for example, by UV laser writingdirectly into the substrate (e.g., glass). The lines are best situatednear the rod where resonator misalignment will have least effect on lineposition.

Picosecond Laser Sub-Harmonic Resonator

In a simple, free running, laser resonator a number of longitudinalmodes develop independently. These modes have no set phase relationshipso they are free to interfere with each other, which leads tofluctuations in the output intensity of the laser as the output signalis an average of all modes inside the resonator.

In frequency space, each mode corresponds to a spectral line and theseparation of spectral lines is called the axial mode interval, c/2 L,where c is the speed of light and L is the optical path length of theresonator (2 L is the round trip optical path length of the resonator).The temporal output of the laser is related to the frequency space by aFourier transform.

Mode locking is a technique used to create pulses of light withdurations less than 1 nanoseconds. This is done by introducing anelement which periodically inhibits the lasing of the resonator. Thisinhibiting element can take a number of forms but the implementation isbroken down into two categories:

(a) Passive mode locking uses an element whose properties are varied bythe light inside the resonator(b) Active mode locking utilizes elements that need to be driven usingexternal signals.

When the mode locking element is a Pockels cell it can be used incombination with a polarizer to vary the losses inside of the resonator.Using the Pockels cell in this manner is equivalent to modifying thereflectivity of one of the cavity mirrors.

The voltage applied to the Pockels cell can be increased until thelasing within the resonator is inhibited. The highest voltage in whichlaser emissions are produced is called the threshold voltage. To modelock the resonator the voltage is modulated around the threshold voltageat a set frequency. When the voltage is lower than threshold the lossesare less and lasing can occur. Voltages higher than threshold willresult in no lasing.

In traditional mode locked lasers the oscillation period of the lasinginhibitor is equal to the time for a pulse to travel one round tripthrough the resonator. Since lasing is inhibited when the Pockels cellvoltage is above threshold a single pulse of light is formed whichpropagates through the resonator. This pulse is formed of longitudinalmodes whose phases are aligned. The peak longitudinal mode will have afrequency which experiences minimal losses when propagating through themode locking element. In the region around this peak the modes willexperience greater loss for greater differences in frequency. Thiscreates a relationship between the longitudinal modes that doesn't existin free running lasers and leads to the smaller pulse durations of modelocked lasers.

A traditional mode locked laser works based on the principal that theelectrical switching frequency at which a mode locker (e.g., a Pockelscell) is switched is directly tied to the optical path length of theresonator. The optical path length of the mode locked resonator canrange from about 3 meters to about 0.5 meters in length, for example.

Active mode locking involves modulation of a component inside theresonator at a frequency whose period is equal to the time required forlight to propagate one round trip in the resonator. The purpose of thiscomponent is to only allow lasing to occur over a portion of this periodand the end result is a single pulse of light traveling within theresonator.

In the case of the traditional picosecond resonator (i.e., thefundamental) the modulation is applied to the Pockels cell whichrequires several hundred volts of modulation in order to produce themode locking effect. The length of the picopulse resonator is limited bythe highest modulation frequency that can reliably be produced at thisvoltage level.

At this point 75 MHz is believed to be the maximum frequency which canbe created which leads to a 2 meter long resonator. A shorter resonatorwould be preferable from a mechanical point of view as the mirrorpositional sensitivity increases as the resonator length increases.

FIG. 15A shows the modulation signal 1599A applied to the Pockels cellin a traditional picosecond resonator having a threshold voltage 1585A.In the presence of this modulation signal 1599A the intensity 1589Abuilds up in the resonator over time.

For example, a resonator having an optical round-trip length of 10 ftrequires an electrical switching frequency of about 100 MHz. The speedof light in air is approximately 1 ft per nanosecond; therefore, theround-trip time of a photon in a 10 ft resonator is about 10nanoseconds. The Pockels cell therefore is switched at about 100 MHz. Inaccordance with a traditional picosecond resonator (i.e., thefundamental) picosecond seed pulses that are generated in the resonatorpass through the Pockels cell one time per electrical switching event.Unfortunately, switching the Pockels cell at 100 MHz is not an optiondue limitations and to issues such as fidelity issues.

In order to resolve such a problem, a sub-harmonic solution may beemployed. The sub-harmonic approach can include (A) divide the Pockelscell switching frequency by a factor of the n^(th) harmonic (e.g., byany power of 2) and/or (B) dividing the optical path length by a factorof the n^(th) harmonic (e.g., by any power of 2). The approaches A and Bwere first tested on a prototype. This test was done whereby atraditional picopulse laser approach to a 75 MHz modulation frequencywould call for a 2 meter resonator length (A) using the switchingfrequency approach the modulation frequency of the existing 75 MHz, 2meter resonator was changed to a modulation frequency of 37.5 MHz. Then(B) using the optical path length approach the 75 MHz modulationfrequency was maintained, but the path length of the resonator wasreduced to 1 meter, which was half the original 2 meter length.Approaches (A) and (B) produced pulses of similar pulse widths to thetraditional 75 MHz and 2 meter resonator length design.

Embodiments Relating to Dividing the Pockels Cell Switching Frequency bya Factor of the n^(th) harmonic (e.g., by any n>1, n is a Whole Number.)

In one embodiment, a system in which the electrical switching frequencyis a sub-multiple of the standard resonator switching frequency isimplemented. In other words, a system is implemented in which seedpulses that flow in the resonator pass through the Pockels more than onetime for every electrical switching event. The modulation signal can beviewed as a gate which allows the light to pass. When the Pockels cellvoltage is below threshold the gate is closed. So a single pulse travelsaround the resonator passing the Pockels cell while the gate is open andall other radiation is suppressed when the gate is closed. Consideringthis analogy, the proposed idea is to close the gate every other roundtrip through the resonator. This would allow for shorter resonatorlengths for a given modulation frequency.

FIG. 15B shows a lower frequency modulation signal 1599B applied to thePockels cell in a sub-harmonic picosecond resonator having a thresholdvoltage 1585B, this is the nth harmonic of the switching frequency offrequency modulation signal 1599A shown in FIG. 15A. In the presence ofthis lower frequency modulation signal 1599B the intensity 1589B buildsup in the resonator over time such that, referring now to FIGS. 15A and15B, at the time of about 140 nanoseconds the intensity inside theresonator 1589A and 1589B is substantially the same.

While we have shown the modulation signal 1599A in FIG. 15A and themodulation signal 1599B in FIG. 15B as featuring an idealized sine wave,in actual usage in the picosecond system the modulation signal has atleast some harmonic content. More specifically, the modulation signalshould have from about 5% to about 50% harmonic content, and from about10% to about 20% harmonic content.

Embodiments Relating to Dividing the Optical Path Length by a Factor ofthe Nth Harmonic (e.g., by n>1, n is a Whole Number).

In another sub-harmonic approach, inhibiting the lasing on every otherpass through the resonator would be sufficient to produce a mode lockedpulse. This sub-harmonic approach can decrease the picopulse resonatorlength and/or ease the electrical burden by decreasing the modulationfrequency.

In a normal mode locked laser a pulse of light propagates one round tripthrough the resonator for each oscillation of the mode locking element.If the element were instead driven at half the frequency, or the firstsub harmonic, then the pulse would travel two round trips for eachoscillation. During the first trip the pulse would travel through theresonator while the element was at maximum transmission. This is thesame in the standard mode locking resonator. During the second trip thepulse will hit the element at minimal transmission and experience loss.If the gain of the active medium is sufficient then the pulse energywill increase more during the first trip than it loses in the secondtrip and a mode locked pulse can be generated.

Since the modulation frequency is tied to the propagation time throughthe resonator, modulating with a subharmonic provides the benefit of ashorter overall resonator length. For example, if the electrical circuitcan reliably switch the required voltages at 50 MHz then the period ofone oscillation is 20 nanoseconds. A 6 meter round trip cavity length isrequired for a travel time of 20 nanoseconds. However, if 50 MHz is thefirst subharmonic of the resonator then the round trip cavity length iscut in half to 3 meters. If we consider the frequency of oscillation tobe a limiting factor then subharmonic operation provides smallerresonators than traditional mode locking.

A method of evaluating mode locked resonators was developed by Kuizengaand Siegman (D. J. Kuizenga and A. E. Siegman, “FM and AM mode lockingof the homogenous laser-Part1: Theory”, IEEE Journal of QuantumElectronics, November 1970, pp. 694-708 [1]) Their analysis applies aself-consistent criterion on the pulse after one round trip of theresonator. Energy travels through an active medium and back then througha modulator and back.

The following expression, Formula (1), relates the pulse width, τ, tothe gain, g, modulation depth, δ, modulation frequency, fm, and gainbandwidth, Δf.

$\begin{matrix}{\tau = {\frac{\sqrt{\sqrt{2}\ln 2}}{\pi}\left( \frac{g}{\delta} \right)^{\frac{1}{4}}\left( \frac{1}{f_{m}\Delta f} \right)^{\frac{1}{2}}}} & {{Formula}(I)}\end{matrix}$

A similar analysis can be done for the sub harmonic resonator, but theself-consistent criterion can only be applied after n round trips of theresonator, n>1, n a whole number. The overall transmission function forthe n round trips must be computed to discover the modulation depthvariable (δ). The sub harmonic overall transmission will be <100% andshows a variation from one round trip to the next during the n roundtrips taken for the analysis.

In one embodiment, a resonator was constructed using an 8 mm Alexandriterod, 85 mm in length and a KD*P Pockels cell. In the first configurationthe Pockels cell is driven at 75 MHz and the path length is 2 meters.This system is operating with the traditional fundamental mode lockingfrequency for this resonator. Pulsewidth of 550 picoseconds are producedby this configuration. The system is then configured to mode lock at thefirst sub harmonic such that it modulates at 50 MHz and the path lengthis decreased to 1.5 meters. Pulses of 700 picoseconds are produced bythis system. Even though the equation at Formula (1) was developed for atraditional mode locking approach the pulse widths of these two systemsreasonably follow the square root of one over the frequency term of theabove expression.

FIG. 16 depicts a representative embodiment of an apparatus 1600according to the present disclosure, which is capable of achieving theabove pulse duration and energy output parameters, suitable for theeffective treatment of pigmented lesions through photomechanical means.Advantageously, the apparatus includes a resonator (or laser cavity)capable of generating laser energy having the desirable pulse durationand energy per pulse, as described herein. The resonator has acharacteristic longitudinal or optical axis 1622 (i.e., the longitudinalflow path for radiation in the resonator), as indicated by the dashedline. Also included in the representative apparatus shown are anelectro-optical device, in this case a Pockels cell 1620, and apolarizing element also referred to as a polarizer 1618 (e.g., athin-film polarizer). During operation, the laser pulse output will beobtained along output path 1623.

At opposite ends of the optical axis 1622 of the resonator are a firstmirror 1612 and a second mirror 1614 having substantially completereflectivity. This term, and equivalent terms such as “substantiallytotally reflective” are used to indicate that the mirrors 1612 and 1614completely reflect incident laser radiation of the type normally presentduring operation of the resonator, or reflect at least 90%, preferablyat least 95%, and more preferably at least 99% of incident radiation.The mirror reflectivity is to be distinguished from the term “effectivereflectivity,” which is not a property of the mirror itself but insteadrefers to the effective behavior of the combination of second mirror1614, Pockels cell 1620, and polarizer 1618 that is induced by theparticular operation of the Pockels cell 1620, as discussed in detailbelow.

In particular, a laser pulse traveling from lasing or gain medium 1616towards second mirror 1614 will first pass through polarizer 1618, thenPockels cell 1620, reflect at second mirror 1614, traverse Pockels cell1620 a second time, and finally pass through polarizer 1618 a secondtime before returning to gain medium 1616. Depending upon the biasvoltage applied to Pockels cell 1620, some portion (or rejectedfraction) of the energy in the pulse will be rejected at polarizer 1618and exit the resonator along output path 1623. The remaining portion (ornon-rejected fraction) of the energy (from 0% to 100% of the energy inthe initial laser pulse) that returns to the medium 1616 is the“effective reflectivity” of second mirror 1614. As explained above, forany given applied voltage to Pockels cell 1620, the effective behaviorof the combination of second mirror 1614, Pockels cell 1620, andpolarizer 1618 is indistinguishable, in terms of laser dynamics, fromthat of a single partially reflective mirror, reflecting the samenon-rejected fraction described above. An “effective reflectivity ofsubstantially 100%” refers to a mirror that acts as a substantiallytotally reflective mirror as defined above.

Also positioned along the optical axis 1622 of the resonator is a lasingor gain medium 1616, which may be pumped by any conventional pumpingdevice (not shown) such as an optical pumping device (e.g., a flashlamp) or possibly an electrical or injection pumping device. A solidstate lasing medium and optical pumping device are preferred for use inthe present disclosure. Representative solid state lasers operate withan alexandrite or a titanium doped sapphire crystal. Alternative solidlasing media include a yttrium-aluminum garnet crystal, doped withneodymium (Nd:Y AG laser). Similarly, neodymium may be used as a dopantof pervoskite crystal (Nd:YAP or Nd:Y Al03 laser) or ayttrium-lithium-fluoride crystal (Nd:YAF laser). Other rare earth andtransition metal ion dopants (e.g., erbium, chromium, and titanium) andother crystal and glass media hosts (e.g., vanadite crystals such asYV04, fluoride glasses such as ZBLN, silica glasses, and other mineralssuch as ruby) of these dopants may be used as lasing media.

The above mentioned types of lasers generally emit radiation, inpredominant operating modes, having wavelengths in the visible toinfrared region of the electromagnetic spectrum. In an Nd:YAG laser, forexample, population inversion of Nd+3 ions in the YAG crystal causes theemission of a radiation beam at 1064 nm as well as a number of othernear infrared wavelengths. It is also possible to use, in addition tothe treating radiation, a low power beam of visible laser light as aguide or alignment tool. Alternative types of lasers include thosecontaining gas, dye, or other lasing media. Semiconductor or diodelasers also represent possible sources of laser energy, available invarying wavelengths. In cases where a particular type of laser emitsradiation at both desired and undesired wavelengths, the use of filters,reflectors, and/or other optical components can aid in targeting apigmented lesion component with only the desired type of radiation.

Aspects of the disclosure also relate to the manner in which theapparatus 1600, depicted in FIG. 16 , is operated to generate laserenergy with the desirable pulse duration and energy output requirementsdiscussed above. For example, laser energy from the lasing medium 1616is reflected between the first mirror 1612 and second mirror 1614 atopposite ends of the optical axis 1622 of the resonator. Laser energyemanating from the lasing medium 1616 therefore traverses the thin filmpolarizer 1618 and Pockels cell 1620 before being reflected by thesubstantially totally reflective second mirror 1614, back through thePockels cell 1620 and polarizer 1618.

Naturally birefringent laser gain materials such as alexandrite, andother crystals such as Nd:YV04 exhibit a large stimulated emissioncross-section selectively for radiation having an electric field vectorthat is aligned with a crystal axis. Radiation emitted from such lasingmaterials is therefore initially linearly polarized, the polarized axiscorresponding to the materials highest gain crystolalographic axis.Typically the polarizer 1618 is configured for transmission ofessentially all incident radiation by proper alignment with respect tothe electric field vector.

Optionally, referring still to FIG. 16 , an astigmatic element 1619 maybe placed anywhere along the optical axis 1622 including, for example,directly in front of one or more mirrors 1612, 1614. Further, one ormore of the mirrors 1612, 1614 can provide an astigmatic element bypossessing two different radii of curvature that are perpendicular toone another.

Referring to the simple apparatus of FIG. 16 . When the laser thresholdbias DC voltage is applied to the Pockels cell 1620 then the effectivemirror reflectivity is set at such a value that the medium 1616 willlase. Varying the voltage above and below the bias voltage is calledmodulating the voltage. In one embodiment, using an Alexandrite medium1616, a typical DC bias voltages applied to Pockels cells is around 650Vand the modulated voltage applied to the Pockels cell is about 200V.

FIG. 17 depicts a representation of the seed pulse grown using asub-harmonic resonator as disclosed herein. The varying amplitude of theseed pulses while operating in the sub-harmonic regime is depicted inthis plot. The trace pulse height variation of repeated high then low isdue to the subharmonic used being an n=2.

The picopulse laser uses a mode locking to achieve its short pulsewidth.The mode locker is a constricting device which is only fully open for asmall fraction of the time it takes a photon to make a round trip in theresonator. So of all the photons circulating and making round trips,only those which arrive at the gate at the right moment will find itfully open, all other photons will experience a loss. Over many roundtrips this elimination of all but the ‘correctly’ timed photons resultsin a shortening of the pulsewidth. All prior literature suggests youdrive open the gate once per round trip or even twice per round trip for2 pulses to be present and so on. The picopulse laser with thesub-harmonic resonator is not run at once per round trip but at once per2 round trips, hence it is sub-harmonic on a single round trip (e.g., inthis example it is one half harmonic).

While the embodiments of the disclosure described herein detail theadvantages of implementing the modified pump chamber of a multi-mode,mode-locked operated laser, one skilled in the art would recognize thatsuch advantages may be experiences using other types of lasers andoperations, such as, for example, multi-mode, non-mode locked operation.

The use of headings and sections in the application is not meant tolimit the invention; each section can apply to any aspect, embodiment,or feature of the invention.

Throughout the application, where compositions are described as having,including, or comprising specific components, or where processes aredescribed as having, including or comprising specific process steps, itis contemplated that compositions of the present teachings also consistessentially of, or consist of, the recited components, and that theprocesses of the present teachings also consist essentially of, orconsist of, the recited process steps.

In the application, where an element or component is said to be includedin and/or selected from a list of recited elements or components, itshould be understood that the element or component can be any one of therecited elements or components and can be selected from a groupconsisting of two or more of the recited elements or components.Further, it should be understood that elements and/or features of acomposition, an apparatus, or a method described herein can be combinedin a variety of ways without departing from the spirit and scope of thepresent teachings, whether explicit or implicit herein.

The use of the terms “include,” “includes,” “including,” “have.” “has,”or “having” should be generally understood as open-ended andnon-limiting unless specifically stated otherwise.

The use of the singular herein includes the plural (and vice versa)unless specifically stated otherwise. Moreover, the singular forms “a,”“an.” and “the” include plural forms unless the context clearly dictatesotherwise. In addition, where the use of the term “about” is before aquantitative value, the present teachings also include the specificquantitative value itself, unless specifically stated otherwise.

It should be understood that the order of steps or order for performingcertain actions is immaterial so long as the present teachings remainoperable. Moreover, two or more steps or actions may be conductedsimultaneously.

While only certain embodiments have been described, it will beunderstood by those skilled in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeas defined by the appended claims. Those skilled in the art willrecognize, or be able to ascertain using no more than routineexperimentation, many equivalents to the specific embodiments describedspecifically herein. Such equivalents are intended to be encompassed inthe scope of the appended claims.

The patent, scientific and medical publications referred to hereinestablish knowledge that was available to those of ordinary skill in theart. The entire disclosures of the issued U.S. patents, published andpending patent applications, and other references cited herein arehereby incorporated by reference.

1.-55. (canceled)
 56. An apparatus for delivery of pulsed treatmentradiation comprising: a pump radiation source generating pumping pulsesat a first wavelength, a wavelength-shifting oscillator defining aresonant cavity, and an optical amplifier, wherein the oscillator outputwavelength varies in response to controlled temperature changes, whereinthe oscillator temperature is controlled to emit a maximum energyamplified output pulse from the optical amplifier at a secondwavelength.
 57. The apparatus of claim 56, wherein the resonant cavityof the wavelength-shifting oscillator has a round trip time shorter thanthe duration of the pumping pulses.
 58. The apparatus of claim 56,wherein the oscillator output wavelength varies with the peakamplification wavelength of the optical amplifier.
 59. The apparatus ofclaim 56, wherein the oscillator output second wavelength varies so thatthe amplified output pulse has minimal pulse duration.
 60. The apparatusof claim 56, wherein the pumping pulses at the first wavelength are subnanosecond pulses.
 61. The apparatus of claim 56, wherein the oscillatortemperature is controlled to range between about 0° C., and about 100°C.
 62. The apparatus of claim 56, wherein the oscillator temperature iscontrolled to range between about 20° C., and about 80° C.
 63. Theapparatus of claim 56, wherein the oscillator temperature is controlledto range between about 30° C., and about 70° C.
 64. The apparatus ofclaim 56, wherein the wavelength-shifting oscillator operates withoutuse of a modelocker or a Q-switch.
 65. The apparatus of claim 56,wherein the wavelength-shifting oscillator emits a subnanosecond pulse.66. The apparatus of claim 56, wherein a lasing medium of thewavelength-shifting oscillator comprises a neodymium-doped crystal. 67.The apparatus of claim 56, wherein a lasing medium of thewavelength-shifting oscillator comprises a solid state crystal mediumselected from the group consisting of neodymium-doped yttrium-aluminumgarnet (Nd:YAG) crystals, neodymium-doped pervoskite (Nd:YAP orNd:YAlO₃) crystals, and neodymium-doped yttrium-lithium-fluoride(Nd:YAF) crystals.
 68. The apparatus of claim 56, wherein a lasingmedium of the wavelength-shifting oscillator comprises aneodymium-doped, vanadate (Nd:YVO₄) crystal.
 69. The apparatus of claim56 further comprising a polarizer embedded within thewavelength-shifting oscillator.
 70. The apparatus of claim 56, wherein alasing medium of the wavelength-shifting oscillator is a polarizingmedium.
 71. The apparatus of claim 56 further comprising afrequency-doubling crystal.
 72. The apparatus of claim 71, wherein thefrequency-doubling crystal comprises a nonlinear crystal material. 73.The apparatus of claim 71, wherein the frequency-doubling waveguidecomprises a lithium triborate (LiB₃O₅) material or a KTP material. 74.The apparatus of claim 56, wherein the pump radiation source is amode-locked laser.
 75. The apparatus of claim 74, wherein themode-locked laser comprises an alexandrite laser.
 76. The apparatus ofclaim 74, wherein the mode-locked laser generates pulsed laser energyhaving at least about 100 mJ/pulse.
 77. The apparatus of claim 56further comprising a treatment beam delivery system configured to applya treatment beam in the form of the second wavelength to a targettissue.
 78. The apparatus of claim 56, wherein the wavelength-shiftingoscillator emits a laser energy having at least about 100 mJ/pulse. 79.The apparatus of claim 56 further comprising a second harmonicgenerator, wherein the pulsed treatment radiation at the secondwavelength from the optical amplifier is transmitted through the secondharmonic generator and is converted to a third wavelength.
 80. Theapparatus of claim 79, wherein the wavelength-shifting oscillatorfollowed by the optical amplifier and the second harmonic generatoremits a laser energy having at least about 100 mJ/pulse.
 81. A method ofdelivering pulsed treatment radiation comprising: generating pumpingpulses, using a pump radiation source, wherein one or more pumpingpulses are at a first wavelength and a first pulse duration;transmitting the pumping pulses to a wavelength-shifting oscillator andthen from the oscillator to an optical amplifier; varying thetemperature of the oscillator such that the oscillator output wavelengthvaries; and controlling the temperature of the oscillator to emit amaximum energy output from the optical amplifier; and emitting pulsedtreatment radiation at a second wavelength from the optical amplifier.82. The method of claim 81 further comprising applying a treatment beamto a patient, the treatment beam comprising pulsed treatment radiation.83. The method of claim 81 wherein the pulsed treatment radiationcomprises a subnanosecond pulse.
 84. The method of claim 81 furthercomprising applying one or more subnanosecond pulses to a patient. 85.The method of claim 81 further comprising applying one or moresubnanosecond pulses to a tattoo or a region of skin pigmentation. 86.The method of claim 81, wherein the optical amplifier emits a laserenergy having at least about 100 mJ/pulse.
 87. The method of claim 81,further comprising a second harmonic generator, wherein the pulsedtreatment radiation at the second wavelength from the optical amplifieris transmitted through the second harmonic generator and is converted toa third wavelength.
 88. The method of claim 87, wherein thewavelength-shifting oscillator followed by the optical amplifier and thesecond harmonic generator emits a laser energy having at least about 100mJ/pulse.